SPECT remains an important technique for assessing myocardial perfusion. However SPECT in general suffers from low sensitivity because of the necessity for collimation. New designs have emerged with 5-8 times the sensitivity of the standard gamma cameras currently used in the clinic for estimating myocardial perfusion. For an overview of the technologies see Slomka et al, “Advances in technical aspects of myocardial perfusion SPECT imaging”, J. Nucl Cardiol, vol. 16, no. 2, pp 255-276, March/April 2009. Most of these designs choose a region of interest around the heart.
The DSPECT system available from Spectrum Dynamics Ltd., 4115 Blackhawk Plaza Circle, Suite 100, Danville, Calif. 94506 uses parallel-hole collimation. Erlandsson et al. (“Performance Evaluation of D-SPECT: a novel SPECT System of nuclear cardiology”, Phys. Med. Biol, vol. 54, pp 2635-2649, 2009) and Gambhir et al. (“A novel High-Sensitivity Rapid-Acquisition Single-Photon Cardiac Imaging Camera”, J. Nucl. Med, vol. 50, No 4, pp 635-643, April 2009) analyzed the DSPECT system which uses 9 flat CZT detectors with parallel-hole collimation arranged in a configuration to conform to the shape of the patient's chest. Each of the 9 detector blocks rotates around its central axis and are also translated to give a complete tomographic sampling. Higher sensitive, worse resolution collimators are used compared to the Low Energy High Resolution (LEHR) collimator used in a standard cardiac acquisition. Hence the geometric resolution was expected to be worse by more than a factor of 2. However, using collimator resolution compensation in iterative OSEM reconstruction the resolution degradation with respect to standard system was entirely compensated for. The planar sensitivity improvement compared to standard GE (Infinia) acquisition was 5.5 times and for tomographic reconstruction the improvement was 4.6-7.9 times for the heart region. The acquisition times reported by Gambhir for clinical studies were 5.5 times faster compared to standard system (2 min for DSPECT versus 11 mins for conventional). The CZT-based system can be used for dual-isotope molecular imaging.
Another system, CardiArc (available from CardiArc, 7444 Haggerty Road, Canton, Mich. 48187, www.cardiarc.com) uses a slit aperture moving over horizontal vanes (effectively achieving slit-slat collimation) over a curved-shaped detector. The CardiArc was designed with semiconductor CZT as well as crystal NaI detectors. Clinical images displayed on the CardiArc website pertain to the CZT design. The acquisition time reported by the company is 2 minutes. The resolution is 3.6 at 82 mm depth of source from aperture.
Pinhole collimation is used in UFC (Ultra Fast Cardiac SPECT Camera) from GE. This system is described in L. Volokh et al., “Myocardial Perfusion Imaging with an Ultra-fast Cardiac SPECT Camera—a Phantom Study”, in Proc IEEE NSS-MIC, Dresden, Germany, pp. 4636-4639, Oct. 19-25, 2008, I. Blevis et al., “CZT Gamma Camera with Pinhole Collimator: Spectral Measurements”, in Proc IEEE NSS-MIC, Dresden, Germany, pp. 4931-4932, Oct. 19-25, 2008, and L. Volokh et al., “Effect of detector energy response on image quality of myocardial perfusion SPECT”, in Proc IEEE NSS-MIC, Dresden, Germany, pp. 4043-4036, Oct. 19-25, 2008. The advantages of pinhole designs are that there are no moving parts, thus reducing manufacturing and servicing costs. The UFC system also uses CZT detectors. Initial UFC performance reports indicate rest and stress acquisition times of 5 and 3 minutes compared to 12.5 and 10 mins for GE Ventri Camera [2] for an anthropomorphic phantom.
Funk et al., “A Novel approach to Multipinhole SPECT for Myocardial Perfusion Imaging”, J. Nucl. Med, vol. 47 pp 595-602, 2006, used a multi-pinhole system attached to a NaI crystal detector. Detailed measurements and simulations were done on point sources and anthropomorphic phantom. Their measurements were compared to parallel-collimation using LEGP (low energy general purpose collimators) and from those measurements it was predicted by simulation that the system would provide sensitivity improvement factor of 5 over the standard parallel-LEHR that is typically used for myocardial perfusion clinical studies for similar resolution. Further they did simulation studies with the NCAT phantom using a single-view, 2-view and 4-view of the 9-pinhole system and found that 4-views (with 36 pinholes) were adequate for artifact-free reconstruction.
The small-animal imaging literature is rich with fine-resolution and/or fast acquisition system designs. A SPECT/CT system is described by A. Stolin et al., “Dual-modality scanner for small animal imaging”, in Proc IEEE NSS-MIC, vol. 4, Oct. 29-Nov. 1, 2006, pp. 2403-2407, 2006. For the SPECT part there are 4 rotating detector heads (with pinholes or parallel collimators) which can be operated in pairs to implement an unique half-cone geometry, reducing the acquisition time compared to a full-cone-beam geometry. Use of a multiplexed coded aperture system as described by R. G. Paxman et al. (“Two Algorithms for Use with an Orthogonal—View Coded-Aperture System”, J. Nucl. Med, vol. 25, pp 1700-1705, 1984) with pinholes projecting into overlapping detector areas also result in more efficient coverage of detector space, leading to an increase in system sensitivity, as discussed in S. R. Meikle et al., “An Investigation of Coded Aperture imaging for Small Animal SPECT”, IEEE Trans. Nucl. Sci., vol. 48, no. 3, pp 816-821, June 2001 and N. U. Schramm et al., “High-Resolution SPECT Using Multipinhole Collimation”, IEEE Trans. Nucl. Sci., vol. 50, no 3., pp 315-320, 2003. However, the multiplexing can degrade the system matrix and can introduce problems in tomographic image reconstruction. F. J. Beekman and B. Vastenhouw, “Design and simulation of a high-resolution stationary SPECT system for small animals”, Phys. Med. Biol, vol. 49, pp 4579-4592, 2004 describes a stationary configuration of pinholes focused on the small object. The performance of a triple-detector SPECT system with 2 pinholes per detector is described in R. E. Zimmerman et al., “Performance of a Triple-Dectector, Multiple-Pinhole SPECT System with Iodine and Indium Isotopes”, in Proc IEEE NSS-MIC, vol. 4, October 16-22, pp. 2427-2429, 2004 and in S. C. Moore et al., “A triple-detector, multiple-pinhole system for SPECT imaging of rodents”, J. Nuc. Med, vol. 45, pp. 97P, 2004.
Recently a new paper [M. A N Korevaar, J. W T. Heemskerk and F. J Beekman, “A pinhole gamma camera with optical depth-of-interaction elimination” Phys. Med. Biol. 54 (2009) N267-N272] has come to our attention where curved detectors fitted to pinholes were used to reduce depth of interaction effects in the detector. Further, for scintillator cameras, in US 2009/0266992 A1, (publication date Oct. 29, 2009), Dr. Beekman proposed replacing light-guides made with scintillator materials with non-scintillator material bundles (such as fiber-optics bundles). To get the best positional information from light-guides, they should be aligned to the pinhole as much as possible and a curved detector was proposed as one of the design examples in US 2009/0266992 A1.
Various descriptions of SPECT cameras, detectors and methods are given in U.S. Pat. Nos. 7,233,002, 6,943,355, 5,311,427, 5,281,821, 5,103,098 and 4,639,599, and U.S. patent application Ser. No. 11/988,947, filed Mar. 20, 2008 and published as US Patent Application Publication No. US 2009/0266992 A1, U.S. Ser. No. 12/083,383, filed Feb. 9, 2009 and published as US Patent Application Publication No. US 2009/0242775 A1, and U.S. Ser. No. 12/225,092, filed Dec. 11, 2008 and published as US Patent Application Publication No. US 2009/0114825 A1. Each of the above-identified disclosures is incorporated herein by reference in its entirety for all purposes.
A number of problems in SPECT cameras relating to resolution and sensitivity have been observed.
There is a need for a SPECT camera that provides improved resolution and sensitivity.